In the mobile telephone standard known as Global System for Mobile communications (GSM), a time division multiple access (TDMA) is used for transmit and receive RF signals. Before the transmit time slot, the RF power amplifier is enabled and it stays on for approximately 588 us until the transmit operation is completed. This scenario repeats every approximately 4.6 ms, and the output signal is then called transmit burst. The operation of the power amplifier requires large DC current to be withdrawn from the battery. In some cases this current can exceed 2 amperes. This current will be switched on and off based on the same TDMA control signals which will cause pulses of large current at frequency of 217 Hz.
Unfortunately, an unintended result of the burst transmission system is a magnetic signal radiating from a mobile telephone employing the GSM standard. The electromagnetic signal has components centered at 217 Hz (fundamental frequency) and centered at frequencies that are multiples (harmonics) of 217 Hz. Other scenarios may likewise result in large current draws from a battery and consequent generation of electromagnetic signals, but the above scenario is a representative situation in which they may occur.